1. Field of the Invention
The present invention relates generally to a plasma welding process. More specifically, the invention relates to a plasma welding process suitable for performing plasma welding for a welding portion of a plate having a thickness of greater than or equal to 6 mm with sequentially varying welding position, such as plasma welding for medium or large diameter stationary pipe, tank or ship and so forth.
2. Description of the Related Art
Plasma welding is a welding method employing a high energy beam as a heat source. A keyhole welding employing a plasma as the heat source has been employed in view of improvement of efficiency.
FIG. 1A is a longitudinal section showing a plasma keyhole welding process, and FIG. 1B is a plan view thereof. In a welding torch 1, a non-consumable electrode 2 is arranged at the center portion. About this non-consumable electrode 2, a plasma gas nozzle 3 and a shield gas nozzle 4 are arranged in coaxial fashion. By densing energy of an arc generated by the non-consumable electrode 2 by passing through the nozzle 3 and passing a plasma gas through the high temperature arc to form a plasma state, a plasma arc 9 is generated by ionization of the plasma gas. The plasma arc 9 is cooled by the shield gas injected from the shield gas nozzle 4 to be restricted spreading and protected from oxidation by the ambient air. The plasma arc 9 is a high energy heat source locally heat a plate 5 to be welded as the base material to form a molten pool 6. On the other hand, by the plasma arc injected at high velocity, the molten metal is depressed to form a keyhole 8. By moving the plasma arc 9 along a groove 10 of the plate 5 to be welded, a plasma keyhole is advanced with melting the base material. Then, the molten metal is moved backwardly to fill the rear side keyhole. Therefore, the keyhole is constantly maintained at a fixed configuration. At the backside of the plasma arc 9 with respect to the moving direction, the molten pool 6 is formed and the molten metal is solidified to form a molten bead 7 at the further backside. The keyhole welding is a most particular high efficiency welding process of the plasma welding which can form a penetration bead as shown in FIGS. 1A and 1B.
Conventionally, in plasma keyhole welding, a pulse current having a peak current value and a base current value is supplied to the electrode. The peak current value, the base current value, frequency and so forth of the pulse current are adjusted for controlling plasma arc, as disclosed in Japanese Unexamined Patent Publication (Kokai) No. Showa 60-27473.
However, this process encounters a problem in that rising period and falling period of respective pulse becomes instant upon control of the pulse current to possibly cause disturbance of the molten pool.
On the other hand, the molten pool 6 has a tendency to become wide at the side of the torch 1 to be so-called wine cup like configuration. Since the width portion of the molten pool is strongly influenced by the current, higher current tends to cause widening of the width of the molten pool rather than deepening the depth of the molten pool. Increasing of the width of the molten pool will not contribute for formation of the keyhole. In case of the upward welding, the widened molten pool possibly causes melting down of the molten pool. Therefore, in the practical plasma arc control method employing the current is not so effective in prevention melting down of the molten pool and in control for all position welding, for example.
In particular, in the plasma welding, the melting configuration tends to become the wine cup like configuration to have large bead width in the vicinity of the surface of the base material to cause increasing of the molten metal amount to increase tendency of drooping down or dropping down of the molten pool. This increases tendency of the phenomenon set forth above.
Namely, in case of the welding for entire circumference of the medium or large diameter horizontal stationary pipe, tank or so forth, the all position welding varying the positions from downward position to upward position across vertical position has to be performed from an initial layer to a final layer. Particularly, when the plasma keyhole welding is applied for the initial layer welding, drooping down of the molten pool is easily caused. Even if the drooping down of the molten pool can be successfully prevented, there is stilled remained a problem to easily cause projecting bead or penetration failure.
As set forth above, in the prior art, all position welding, in which the welding position is varied from the downward position to the upward position across the vertical position, should be performed over the initial layer to the final layer to make setting of the welding conditions complicate.
In case of welding of the stationary pipe, particularly the horizontal stationary pipe, the welding operation is typically performed by an upwardly advancing welding progressing welding from lower portion to the upper portion or the all position welding progressing welding from downward position to upward position across the vertical position and subsequently from the upward position to the downward position across the vertical position. In such case, when the initial layer is welded by arranging the welding torch at the outside of the pipe to perform welding with directing the torch toward external surface of the pipe to be welded, the penetration bead can becomes excessive at the portion near the upper portion of the pipe. Conversely, lack of penetration can be caused in the vicinity of the lower portion. Therefore, it is difficult to stably form a uniform bead. In such case, while the plasma arc welding can obtain deeper penetration depth in comparison with other welding processes and thus is efficient, difficulty is encountered in stabilizing the arc.
In the conventional plasma arc welding employing a pure argon gas as a plasma gas (center gas), the stability of the arc is insufficient in the case of circumferential welding of the stationary pipe. In addition, in the circumferential welding for the stationary pipe, the plasma arc welding tends to cause a blow hole at the bottom portion of the welding bead to cause difficulty in obtaining excellent welding portion. This is particularly remarkable in the case of non-keyhole welding, in which the keyhole is not formed.
In contrast to the keyhole welding, the non-keyhole welding (soft plasma welding) is employed in welding without forming the penetration bead. In such non-keyhole welding, the pure argon gas is typically used as the plasma gas, In such conventional plasma arc welding using the pure argon gas as the plasma gas, a plasma jet has a tendency to be captured in the molten pool to cause blow hole in the bottom of the bead. In the worst case, the plasma jet captured in the molten pool may form a tunnel like defect. Therefore, a difficulty is encountered in obtaining excellent welding portion. This phenomenon is remarkable in the case where the flow rate of the plasma gas is greater than or equal to 1.0 l/min.
On the other hand, in view of the surface property of the base material, when the base material which is processed by machining or grinding to remove surface scale and cleaned, such as a steel plate, is welded by the plasma arc welding, relatively stable and less defective molten metal can be obtained. However, when the surface is not cleaned, for example, in the case where the scale is remaining, the arc can be disturbed by the affect of the iron oxide and easily forms the defects, such as the blow hole in the molten metal.